Soils of Africa
Soil Order Map of Africa
Other Maps of Africa : Country Map - Country Map2- Areas at Risk- Old Soil Order Map - Soils Orders of Tropical Africa- Savanah- Arid Savanah - Sub-humid Savanah Geography 1- Geography 2- Population Density -
AN ASSESSMENT OF THE SOIL RESOURCES OF AFRICA IN RELATION TO PRODUCTIVITY
Hari Eswaran *, Russell Almaraz, Evert van den Berg, and Paul Reich World Soil
Resources, Soil Survey Division, USDA Natural Resources Conservation Service,
Washington D.C. 20013. February, 1996. Corresponding author
(heswaran@usda.gov).
Abstract
Africa, with a total land mass of about 30.7 million km2 and a population
exceeding 746 million persons, has generally lagged behind in agricultural
development. Sub-Saharan Africa (excluding South Africa) is the poorest
developing region, with 29 out of 34 countries being some of the poorest in the
world. Prime land occupies about 9.6% of Africa and the lands with high
potential occupy an area of about 6.7%. The medium and low potential lands,
which together occupy 28.3% of the area have major constraints for low-input
agriculture. Resource poor farmers who live on these lands have high risks and
generally, the probability of agriculture failure is high to very high. The
remaining about 55% of the land consists of deserts or other lands with major
constraints even for low-input agriculture. The desert margins have nomadic
grazing which with increasing animal population is stressing the environment. A
soil quality analysis and an evaluation of sustainable production, based only
on biophysical considerations, suggest the need for major investments to
enhance the productivity of the soil resources of this continent. Index words:
Africa, land productivity, land quality, soil constraints.
http://soils.usda.gov/use/worldsoils/papers/desertification-africa.html
SOIL RESOURCES
Two of the early efforts of compiling the soil resources of Africa, were those
of Marbut (1923) and Shokalskaya (1944). These maps at a scale of 1:30 million
are based on geology and phytogeography and had minimal field soil information.
Many of the recent concepts of tropical weathering and soil formation
originated in Africa during the colonial period (Ollier, 1959; King, 1962, and
Moss, 1968).
In 1953 an Interafrican Pedological Service was created with its Administrative Headquarters at Yangambi, Belgian Congo (Zaire). The Council of this Service decided in 1955 to make a soil map of the continent which was eventually realized in 1964 (D'Hoore, 1964). This soil map and accompanying report was perhaps the last effort in Africa until the FAO-UNESCO Soil Map of the World was published a decade later. In retrospect and particularly realizing the conditions under which soil survey investigations were conducted during the fifties and sixties in Africa, the map and report is a most commendable effort. As will be shown later, soil names may have changed and polygon boundaries refined, but the basic information is still very reliable. As an example, the Vertisols were reported to occupy about 3.28 % of the area; in the present study, the area is 3.23 %. .
Histosols (Eswaran, 1986) is one of the smaller soil Orders in terms of areal distribution, occupying about 0.05 % of the land mass. They are confined to the coastal plain of West Africa and Madagascar where they are associated with acid sulphate soils (Sulfaquents and Sulfaquepts) and other Aquepts and Aquents.
Spodosols are also present as small enclaves in two locations. A large extent is with the wet sandy soils present in depressions in southern Zaire. The sands are wind-blown material from the Kalahari and the soils form a catenary sequence with deep Psamments on the plateaus, Arenic Palustalfs on the slopes leading to the Aquods and Humods, associated locally with Tropofibrists. A small area of Spodosols (Haplohumods) is also found east of Cape Town in S. Africa on wind-blown sands.
Andisols are the third group of soils found in small areas and occupy less than 0.16 % of the area. The upper slopes of Mt. Cameroon, Kilimanjaro, Mt. Kenya, and parts of the Ethiopian Highlands have Andisols.
Oxisols (Eswaran et al., 1986) occupy about 3.75 million km2 or 14.3 % of the land area and are confined to the Congo Basin and adjoining areas. Much of the early information on African Oxisols was provided by the soil survey program of the Belgians (Tavernier and Sys, 1965). Both in the legend of the Belgian maps and that of the FAO-UNESCO Soil Map of the World (FAO-UNESCO, 1977), Oxisols are referred to as Ferralsols, with a definition very similar to the Soil Taxonomy definition. The southern extension of Oxisols in Zambia is not well defined due to the Holocene cover of the Kalahari sands which are of varying thickness. When the cover is thicker than 150 cm, the soils are classified as Alfisols, Ultisols, or Entisols. Madagascar has a very large extent of Oxisols on the eastern piedmonts and much of the central highlands. These are formed on old basement rock complexes and weathering may extend to several tens of meters. The organic matter rich Oxisols are located in the highlands around Lake Kivu (Rwanda and Burundi) and the north eastern part of Zaire, in the Ituri Province. Many of them have more than 16 kg/m/m2 of carbon qualifying them for humic subgroups. Many also have a sombric horizon which has not been described in other parts of the world. These high organic matter isothermic Oxisols are very productive and support some of the highest population densities in the world.
The Vertisols (Wilding and Puentes, 1988) are distributed along the rift valley from Sudan in the north to South Africa in the south, with sporadic occurrences in other parts of Africa. In North Africa, particularly in Morocco, the Xererts or Tirs have been used for agriculture since the Roman period. They are extremely difficult soils for tillage (Eswaran et al., 1988) both in the wet and dry season and until the recent introduction of high energy equipment, much of the cultivation was confined to the post-rainy period. On the African Rift, the Vertisols of Sudan on the Gezira Plains are the best known and are most intensively used under irrigation. There are also large contiguous areas in South Africa, Zambia, Kenya, and Somalia, and occur locally in West Africa.
Aridisols (Eswaran et al., 1993) occupy about 26.4 % of Africa and associated with these soils are the 'torric' great groups of Entisols and some other Orders. The Torripsamments and Torriorthents (Table 2) together occupy about 15 %. The remaining 46.2 % of the areas with aridic SMR of Africa are comprised of rockland, salt pans, sand dunes, and minor components of other 'torri' great groups. Thus for most practical purposes, about half of Africa has land unsuitable for low-input agriculture.
Closely related to the Oxisols are the Ultisols occupying about 6.2 % of the land area (Table 2). In Zambia and the western part of Angola, many of the Ultisols are actually buried Oxisols buried under the Kalahari sands and thus characteristically have very sandy top soils and a low activity clayey subsoil. A similar feature is seen in the Alfisols, which occupy about 10.5 % of the area and are extensive in the Sahelian part of Africa. In the Sahel, which borders the Sahara, the wind-blown sand has buried many of the former Oxisols and Alfisols/Ultisols. In Central Africa, the Alfisols and Ultisols are the typical soils of the mid- and end-Tertiary plateaus. Most of the soils are reworked and some have stone-lines composed of quartz or petroplinthic gravel (Ruhe, 1956). Typically, the geomorphic surfaces occupy specific elevations and at the edge of the plateaus are bands of re-cemented petroplinthite. These are the Petroferric subgroups of some Alfisols and Ultisols. The petroferric contact is an impermeable layer to both roots and water. The Mollisols are more dominant in the areas with xeric SMR, with large extents in Morocco and the coastal areas of Algeria and Tunisia.
In sub-Saharan Africa, the Mollisols are confined to the isothermic areas on recent base rich materials. In these areas, a Mollisol-Alfisol association is common. Inceptisols occupy about 7.8 % and Entisols, about 24.5 % of the land mass of Africa. About 50 % of the Entisols have an aridic SMR and are formed on sandy or loamy deposits. Another about 5 % of the Psamments are present as interfingerings of the Kalahari or Sahara in zones with ustic or udic SMRs. A large proportion of the Inceptisols are shallow soils on dissected or mountainous lands. They are generally not used for agriculture.
SOIL QUALITY
Constraints to agricultural use Soil quality is the ability of the soil to
perform its functions in a sustainable manner. For detailed land use
assessments, quantitative soil quality assessments can be made. As the intent
here is to make qualitative continent-level assessment, only factors that
reduce the ability of the soil to perform in an optimal manner for agriculture
are considered. Constraints to sustained use of soils are multiple and include
socioeconomic constraints which are not considered here. Only the biophysical
constraints are evaluated with the objective of having some continent-level
estimates.
Soil moisture stress is the overriding constraint in much of Africa. Only about 14 % of Africa is relatively free of moisture stress. The other major stresses discussed here relate to soil properties and are considered individually, though the same soil may have more than one of these constraints. Moisture stress is not only a function of the low and erratic precipitation but also of the ability of the soil to hold and release moisture. About 10 % of the soils have high to very high AWHC. These are mainly the Mollisols, Vertisols, and other clayey soils with 2:1 layer lattice clays. The 29 % of soils with medium AWHC are mainly the Alfisols and Ultisols and some loamy Inceptisols and Entisols. The low AWHC class soils are the Oxisols and other sandy loam soils. Despite their clayey textures, Oxisols have low AWHC. The very low AWHC class soils are the sandy soils such as Psamments and other sandy and sandy loam soils.
The potential of soils to fix phosphorous is difficult to estimate. High free iron content, which is reflected in the red colors of the soil, due to the nature of the parent material or the weathering stage, is employed as an indicator. The high P fixing soils are mainly the sesquioxide rich Oxisols and Ultisols. P is immobilized as Fe- and Al- phosphates in these soils. This is a crude estimate of P in soils and a refined evaluation would require determination of the activity of P in the soil. In the arid and semi-arid parts of Africa, salinity and alkalinity is a major problem affecting about 24 % of the continent . These are included in the soils designated as having a pH >8.5 in table 3d. The extremely acid soils, which are mainly the acid sulphate soils (both potential and actual) occupy a small area around the Niger delta and occur sporadically along the coastal plains of West Africa. The soils which have high aluminium problems, occupy about 15 % of the continent and are mainly in the moist parts of the semi-arid zones and the sub-humid areas. Many of the Ultisols and some Alfisols have acid surface and subsurface horizons which, coupled to the moisture stress conditions, makes these soils extremely difficult to manage under low-input conditions.
In West Africa, the annual additions of dust from the Sahara brought by the Harmattan winds, raise the pH of the surface horizons and so the problem is less acute but subsoil acidity remains. Soil depth is frequently understood as being depth to rock or an impermeable layer. The term effective soil depth is used here to include chemical barriers which reduce the volume of the soil for root exploitation. Effective soil depth is a problem in more than 50 % of the soils and this reduces the potential of the soil for crop production.
Wind and water erosion is extensive in many parts of Africa. Excluding the current deserts which occupy about 46 % of the land mass, about 25 % of the land is prone to water erosion and about 22 %, to wind erosion. High intensities of these erosion forms are mainly in the semi-arid and sub-humid areas. The soils in Central Africa are largely low activity Oxisols and Ultisols and are less susceptible to water erosion, unless severely mismanaged. These estimates include human-induced erosion, good estimates for which are available in Oldeman et al., 1992. Based on this analysis and with an understanding of the soils and their distribution, it is possible to make an estimate of the lands which are susceptible to desertification, in its broadest sense. About 30 % of Africa, mainly in the semi-arid and sub-humid parts, is highly susceptible to desertification. From this point of view, these are fragile ecosystems and considerable damage can be done with low-input agriculture.
Digital Maps of Africa http://informatics.icipe.org/databank/maps.htm
African Soil Order Map http://informatics.icipe.org/images/afrorder.gif
Morocco http://www.iao.florence.it/training/remotesensing/BenSlimane/Marocco21_3_4_S.htm
Southern Africa Soil Map http://www.uni-koeln.de/inter-fak/sfb389/e/e1/e1_download.htm
Soils Soil Classification Several different systems have been used for classifying the soils of various African countries. These are too numerous to detail, but the French system (Aubert 1968) and the Belgian system (Sys et al. 1961) are two of the most important. Useful reviews of the soils of West Africa are contained in Ahn (1970) and Jones and Wild (1975). East African soils are reviewed by Scott (1972). The FAO soil map of Africa (D'Hoore, 1964) and the African section of the later FAO and Unesco (1974) soil map of the world represent attempts to draw together the available information on African soils.
Sanchez (1976) gives a good summary of information to that date on tropical soils. Although considerable research and detailed mapping have been carried out on the soils of several countries, there appears to be a need for a much greater and more sustained research effort to classify and determine the characteristics of African soils, and particularly to work out improved methods of soil management and conservation that can be applied by smallholders.
Because the US system of soil classification (that is, soil taxonomy, USDA 1975) seems to be gaining increasing acceptance worldwide, the map of African soils used here (Fig. 5) represents a first approximation of the application of this system to African soils (Aubert and Tavernier 1972).
Although considerably more work needs to be done on the adaptation of the soil taxonomy classification system to African soils, there appear to be substantial advantages in its use to allow comparisons between African soils and other similar soils worldwide. Although a map on this large a scale obviously has a limited value for any detailed discussion of agricultural uses, certain broad generalizations may be attempted following Sanchez (1976) and Ahn (1970, p. 220).
Because rainfall and soil moisture are critical for tropical farming systems, as has already been stated, the soil taxonomy system uses the following terms that are important for savanna soils: * Udic — On average, water stress is absent during most of the year; * Ustic — A strong dry season of 3–6 months; * Aridic — A longer dry season: dry and desert climates; and * Aquic — The soil is saturated with water long enough to cause reduced soil conditions.
Alfisols: The map indicates that the soils over vast areas — estimated at 550 million ha (Sanchez 1976, p. 73) — of the African savanna are classified as alfisols. These are a varied group. They have an argillic (clayey) horizon with more than 35% base saturation. They are similar to ultisols except for a considerably higher natural fertility level.
Oxisols: Oxisols are widespread in the high rainfall areas of the Congo basin and extend through Zambia, Mozambique, and neighbouring countries into Madagascar. They are estimated to cover roughly the same area as the alfisols, about 550 million ha. These highly weathered soils are defined as having oxic horizons (above 16 meq/100 g clay), consisting of mixtures of kaolinite, iron oxides, and quartz, that are low in weatherable minerals. These soils are usually deep, well-drained red or yellow soils with excellent granular structure, very low fertility, and uniform properties with depth.
Ultisols: Ultisols are mainly found in relatively humid areas such as Guinea, Liberia, Uganda, and parts of eastern Zaire. They are estimated to cover about 100 million ha. Ultisols are defined as having an argillic horizon (20% increase in clay content in the control section — roughly the main rooting zone) with less than 35% base saturation in this zone. They are usually deep, well-drained red or yellow soils, higher in weatherable minerals than oxisols, with less desirable physical properties, and relatively low natural fertility. Ultisols may have oxic horizons above or below the argillic.
Entisols: Entisols are widespread in the drier areas of south-west Africa, stretching northwards through Angola into Zaire and covering roughly 300 million ha. They are pedologically young soils in which the horizons are only slightly developed or undeveloped. They include many recently deposited alluvial materials and some young soils on inert and resistant parent materials. Inceptisols: The inceptisols occur mainly in the Central African Republic and part of the southern Sudan with an estimated area of 70 million ha. They are slightly more developed than the entisols with a cambic horizon but no other diagnostic horizons, and are also relatively young. They are not strongly weathered, and include some poorly drained gley soils without well-developed horizons, and some volcanic soils.
Vertisols: The largest area of vertisols is in the Sudan and parts of Ethiopia, however, they are fairly widely distributed, mainly in valley bottoms, throughout Africa. Their area is estimated at 40 million ha. These are a unique group of soils, often described as black cotton soils. They are heavy, usually dark coloured, clay soils that form deep cracks when they dry out. Because of the high clay content (over 35% clay containing over 50% of 2:1 — mainly montmorillonitic — clay minerals), they are difficult to cultivate when dry, and become very sticky when wet. Aridisols: As the name indicates, these are soils of arid regions such as the Sahara and Kalahari deserts. At 840 million ha, they cover the largest area of the soil groups in tropical Africa, but because much of this is desert they are less important in the savanna areas. They show some horizon differentiation.
Geomorphology --Although the soil map (Soil Order Map) attempts to show some of the dominant soil types, it represents an oversimplification of a more complex situation. Most of Africa is underlain by a vast shield of ancient Precambrian granitic rock. This shield has been lifted up and eroded down repeatedly over geological time, and parts of it have been submerged under seas or lakes for varying periods, leaving sediments over the granite. As a result, Africa has a very varied topography, but many areas could be described as having a rolling landscape. These repeated hills and valleys have characteristic patterns of soils, depending on the underlying parent materials.
Milne (1935), working in Tanzania, proposed the term catena for these patterns of soils, which often repeat themselves up and down the hills over thousands of kilometres. Gravelly red and brown upland sedentary soils are found at A and B on summits and upper slopes. These grade downslope into yellow-brown, sandy, light, clay soils developed in middle-slope colluvium at C and into yellow-brown, sandy loam and loamy sand colluvium at D. The soils developed in local granite-derived alluvium at E are mostly grey to white sands with subordinate areas of gritty or sandy grey clays. This diagram indicates how the granitic soils on the hilltops with characteristic ironstone gravel at varying depths have been eroded downhill forming the colluvial soils on the slopes and the grey or white sandy or grey clayey alluvial soils in the valley bottoms. Needless to say, there are innumerable variations both in the underlying materials and in these patterns of soils.
Savanna Soil Characteristics-- Kowal and Kassam (1978) have commented in detail on the characteristics of the soils of the West African savanna. The following brief summary draws heavily on their synthesis. Soil Depth and Structure: Kowal and Kassam (1978) emphasize the critical importance of soil depth, because of the widespread presence of ironstone gravel or plinthite in many savanna soils, which can restrict root growth at varying depths and interfere with cultivation. They also stress the sandy nature of most savanna soils, although some of the leached alfisols and ultisols contain increasing amounts of clay, mainly kaolinitic, in the deeper horizons. They point out the importance of soil structure and surface characteristics, particularly when the vegetative cover is removed, so that the soil surface is exposed to heavy rain, and capping (crusting) often takes place. Jones (1987, p. 12) also noted soil capping as a major problem on apparently similar soils in Botswana.
Many savanna soils are fine sandy loams, with high bulk densities that can restrict root growth. On these soils, plowing or deep digging gave crop yields between 20–70% higher than those obtained after shallow hoeing, with an average increase of 24% (Charreau 1974; see also Pingali et al. 1987, p. 62). Soil Moisture: Soil moisture deficiency is one of the primary limiting factors to crop growth in the drier savanna areas. Even in the more humid areas, the seasonal distribution of rainfall described earlier is an important determining factor for crop growth, depending on the moisture-storage capacity within rooting depth of the particular soil. This depends on the depth, organic matter (OM) content, structure, and texture of the soil, particularly the clay content. Moisture storage in the top 1 m of soil varies from about 80 mm in sandy soils to about 150 mm in soils with a higher clay content. Time of planting and crop-growth duration need to be closely fitted to the expected moisture supply for the rainy season. Ways in which farmers can make these adjustments are discussed in more detail in later sections.
Cation Exchange Capacity: The effective cation exchange capacity (CEC) of many savanna soils is low, often below 4 meq/100 g, and is closely related to the OM and clay contents of the soils, many of which have a variable charge (Sanchez 1976, p. 155). CECs can be increased by liming acid soils and increasing OM contents. The implications of this and other changes in soil reaction (pH) are discussed further in a later section.
Organic Matter: Organic matter is of critical importance in savanna soils. Under a thick cover of forest, bush, or even grass fallow, OM increases, the soil surface is protected, and soil temperature is moderated, the CEC is maintained, soil organisms such as earthworms are active (Lal 1983, p. 20), and within the limits of the soil's inherent fertility, productivity is high. Once the vegetation is burned, or the soil is cultivated, soil OM breaks down rapidly, depending on the moisture content. The rate of breakdown is proportional to the temperature (Birch and Friend 1956). Lal (1983, p. 14) and others (for example, Sanchez 1976, p. 105) have emphasized the critical role of mulching in tropical crop production. Mulches decrease the energy of raindrops and therefore protect the soil from capping and erosion, they limit evaporation and therefore retain soil moisture, reduce the rate of OM decomposition, prevent excessively high soil temperatures, which can impede or prevent seedling emergence, increase soil water storage, and decrease weed infestation. As they decompose, mulches make OM and nutrients available for crop growth.
In many areas, farmers traditionally practice mulching using crop residues or uprooted weeds. Crop yields can often be substantially increased by mulching (Table 2). Table 2. Effect of mulch and fertilizer on yield (kg/ha) of cotton in Zaire. Year Without fertilizer With fertilizer Unmulched Mulched Unmulched Mulched 1947–48 1953–54 1955–56 1956–57 1032 200 186 124 1127 1117 1464 986 – 440 797 706 – 1434 1977 1344 Source: Adapted from Jurion and Henry (1969). Kowal and Kassam (1978, p. 103) point out that the effects of OM on soil fertility in tropical soils continue to arouse considerable controversy. However, it appears clear that, on many tropical soils, the addition of OM can give substantial yield increases, and can allow economies in fertilizer applications.
In recent years, there has been a considerable increase in so-called "Organic Farming" throughout the world. Farmers using this method aim to maintain or even increase the OM content of the soil by applying manures, crop residues, and mulches. In this way, they claim to be able to reduce their need for inorganic fertilizers. Some of them refuse to use "artificial" fertilizers, pesticides, and other products. There seems little doubt that when these methods are applied consistently over long periods, soil fertility does build up, and crop yields are often improved. On the other hand, the soil nutrient balance-sheet must be maintained. If the soil is inherently low in nutrients, it may take very large quantities of OM to provide for the needs of heavy yielding crops, particularly if these crops are sold off the farm so that most of their residues cannot be returned to the soil. Provision of this OM can be very laborious and expensive for the farmer. To help overcome this problem, some "organic farmers" are willing to use "natural" fertilizers such as phosphatic rock.
In some western countries, householders worried about the effects of pesticide residues in food are willing to pay increased prices for "organically" grown foodstuffs. These higher prices can give an incentive to farmers to use "organic" methods. Because of the labour and expense involved in providing either sufficient OM or fertilizers for many savanna farming systems, it is considered of critical importance that more research should be directed to the investigation of appropriate combinations of added OM and fertilizers for tropical crop production.
Nutrients-- Phosphorus: Phosphorus deficiencies are widespread in savanna soils, and many authorities consider lack of phosphorus to be the primary nutrient limiting crop production in the savanna (Kowal and Kassam 1978, p. 143), although nitrogen may be the limiting nutrient in the wetter savanna areas (Sanchez 1976, p. 184). Average phosphorus contents in surface soils range from about 80–150 ppm, which are very low compared with temperate soils that have average contents of about 1500–3000 ppm. Part of the soil phosphorus is in the form of organic phosphorus, but this tends to be low in most savanna soils (15–60 ppm, Goldsworthy and Heathcote 1963). Losses of phosphorus from soils by leaching are generally low, but much of the soil phosphorus is held by the soil in various ways making it only slowly available to crops. However, phosphorus fixation is not usually a serious problem in the more sandy savanna soils, although it can be a constraint in some of the heavier acid soils in East Africa and elsewhere.
Many African countries have naturally occurring deposits of phosphatic rock, and some of these deposits are being developed for both local use and export. For example, Senegal and Togo export considerable quantities of phosphate, and Senegal uses some of its production locally. These phosphatic rock deposits vary greatly in the percentage of phosphorus and in its form and availability. The location of some of the deposits is remote, and this determines the ease of extraction and transportation. For example, Mali has started developing a deposit at Tilemsi in the Sahara desert north of Gao, and trials have shown that finely ground phosphatic rock from this deposit can be applied directly to crops with promising results, particularly on the more acid soils. The deposits in Burkina Faso, Niger, and Togo, on the other hand, appear to require processing to increase solubility before use. Because phosphorus, unlike nitrogen and other nutrients, does not move much in the soil, and it is especially required for early root growth, it is important to ensure that phosphorus fertilizers are placed close to seeds, particularly in soils that fix phosphates. This can be done by using a placement drill for planting, or phosphate rock can be mixed with the seed for hand-planting. (Some types of fertilizer may damage the seed or young plants if placed in contact with the seed at planting. These can be put into the planting hole and covered with soil before planting the seed.)
Nitrogen: Nitrogen deficiencies are widespread in savanna farming systems, and are often limiting factors to crop growth (Sanchez 1976, p. 184). The most important source of nitrogen is from biological nitrogen fixation (BNF) from the atmosphere. When leguminous plants or crops are present, Rhizobium bacteria symbiotic in the roots are the main source, but other sources include free-living nitrogen-fixing bacteria in the leaf canopy, the litter, and the rhizosphere, and bluegreen algae, particularly in flooded rice fields. Traditional cropping systems where cereals such as sorghum or millet are intercropped with leguminous crops such as cowpeas or groundnuts appear to assist in reducing nitrogen deficiencies in one or both crops (Greenland 1985, p. 19). The nitrogen status of unfertilized soil is closely associated with the soil OM, and therefore varies with the OM content. Also, Hardy (1946) and Birch (1958) showed that the alternate wetting and drying associated with the first rains produce a flush of mineral nitrogen in a previously dry soil containing OM, because of the action of microorganisms as the OM decomposes. This important phenomenon is sometimes called the "Birch Effect." The amount of mineralized nitrogen varies, but it can be as much as 70 kg N/ha after manuring.
Unlike phosphorus, nitrogen can be lost rapidly from the soil by leaching or by volatilization from the soil surface. These losses can be reduced by early planting of crops, so that the crops can take up some of the mineralized nitrogen before it is lost by leaching. Volatilization can be reduced by incorporating the fertilizer into the topsoil. Another way of reducing the losses is by careful timing of fertilizer nitrogen applications, usually about 2–6 weeks after planting (Charreau 1974).
Potassium: Although potassium deficiencies have been reported on certain soil types under heavy cropping, in general they do not appear to be limiting to crop production in most savanna regions (Nye and Greenland 1960, p. 99). Like phosphate, potassium is returned to the soil in crop residues or other forms of OM, and in the ash after burning, so it is easily recycled, but leaching losses can be severe. Also, although savanna soils are often comparatively low in potassium, the nutrient appears to be readily available in most tropical soils. However, under continuous heavy cropping with the removal of crop residues, potassium could be expected to become limiting to the yields of a number of crops, depending on the CEC and pH of the soils, therefore a careful watch needs to be kept for deficiency symptoms. This does not imply that the uneconomic use of compound fertilizers containing potassium as well as nitrogen and phosphate is justified, however, unless there is strong evidence of the likelihood of potassium deficiencies.
Acidity, Aluminum Toxicity, and Calcium and Magnesium Deficiencies: Many savanna soils are acid, particularly in the more humid areas, and in these soils aluminum toxicity is a major limiting factor to the growth of susceptible crops. These crops include many leguminous species such as groundnuts and soybeans. Maize, sorghum, and many others are also susceptible to varying extents. In contrast, crops such as cassava and coffee can tolerate a certain degree of acidity. The low CEC of most savanna soils limits the amount of exchangeable calcium and magnesium to about 0.3–3.0 meq/100 g. In crops growing on soils in dry areas, it is often difficult to separate the detrimental effects of lack of calcium or magnesium as nutrients, and the indirect effect of aluminum toxicity (Kowal and Kassam 1978, p. 148).
Under traditional shifting cultivation on soils with pH above about 5.5, there is little evidence of much crop response to liming (Nye and Greenland 1960, p. 97). On more acid soils, and on savanna soils under more intensive cropping with heavy use of nitrogenous fertilizers, the pH may often fall below 5.5, and liming may be necessary, although surprisingly few experiments appear to have been conducted on this important topic in the African savanna. Small applications of 1–2 t/ha of lime once in 5–10 years often appear sufficient to raise the soil pH to 5.5, and to prevent aluminum toxicity, and they may give quite large yield responses, particularly in maize, sorghum, and legumes such as groundnuts. For example, it has been estimated that about 20% of the cultivated land in Senegal needs liming to correct acidity and prevent aluminum toxicity (Pieri 1974). On the other hand, overliming the soil to above pH 5.5–6.0 can reduce yields by lowering the availability of phosphate and other nutrients (Sanchez 1976). Most African countries have lime deposits that could be used for this purpose. Although the costs of extraction and transport could be quite high for subsistence cultivators remote from the sources, as cropping intensities increase it would appear essential that greater attention should be paid to this type of amendment.
Sulfur: Sulfur is mainly contained in OM and, like nitrogen, it is lost to the atmosphere when crop residues or bush or grass fallow are burned. Because OM levels are generally low in savanna soils, sulfur deficiencies are widespread, particularly in areas remote from the sea or from smoky industries, where the rain usually contains some sulfur (Nye and Greenland 1960, p. 97). Where fertilizers containing sulfur — such as ammonium sulfate or single superphosphate — are used, little sulfur deficiency occurs, but the increasing use of low sulfur fertilizers — such as urea and triple superphosphate — can result in sulfur deficiency in susceptible crops. Small dressings of up to 20 kg S/ha are sufficient to correct this (Kowal and Kassam 1979, p. 148).
Micronutrients: Occasional deficiencies of micronutrients such as boron, molybdenum, and zinc have been identified, particularly on plantation crops, and on cotton and groundnuts under heavy cropping. Boron is sometimes routinely added to cotton fertilizers, but care must be taken to avoid toxicity, because the margin between deficiency and toxicity is narrow. Manganese is also an essential nutrient, but it is more likely to become toxic, like aluminum, on certain acid soils. This can be corrected by liming (see above).
Soil Erosion It is generally recognized that the greatest threat to sustained rainfed agricultural production, and indeed to the continuing prosperity of many of the developing countries in Africa and elsewhere, is probably uncontrolled soil erosion (Hudson 1981). For example, Brown and Wolf (1985) have suggested that about 1 billion t of top soil are being lost by erosion each year in Eritrea and Ethiopia alone. Although the basis for such estimates is not clear, there is no doubt that uncontrolled erosion is causing enormous damage to the soils of Eritrea, Ethiopia, and most other African countries. As population and land pressure increase, uncontrolled erosion is depleting valuable topsoil, undermining roads and other structures, and causing floods and silting of reservoirs throughout Africa. Yet surprisingly little scientific data on erosion is available from large areas of Africa. Kowal and Kassam (1978) have attempted to summarize the situation in the West African savanna. They point out that, in the higher rainfall areas of the Guinea and Sudan savannas, water erosion is the most important cause, whereas in the Sahel, although water erosion can be severe in the southern subregion during the occasional storm, wind erosion also occurs and is particularly severe in the north.
Erosion Caused by Rain: Under natural vegetation undisturbed by humans or domestic animals, the soil surface is normally protected by leaf litter and a canopy of leaves, and negligible erosion takes place. For example, Roose (1967) estimated the annual rate of soil loss under natural vegetation at Sefa, Senegal (1150 mm rainfall), as 0.1–0.2 t/ha. At Samaru, Nigeria, with a similar rainfall, Kowal and Kassam (1978) found that runoff and soil loss were negligible even when annual burning of natural vegetation was practiced. Because soil formation does occur, albeit very slowly, under savanna conditions, it is evident that the rate of erosion under natural vegetation is unlikely to be as great as the rate of soil formation, so a net gain in top soil occurs. Erosion damage under traditional shifting-cultivation practices with low population pressures of humans and livestock is generally considered to be relatively limited (Sanchez 1976, p. 363). Shifting cultivators usually open small plots for 2–3 years, leaving the stumps of bushes and trees to regenerate during the fallow period, and maintaining a cover of vegetation or crops on the land during most of the rainy season. Even if some runoff and erosion does occur within the plot, it is usually arrested as soon as it reaches natural vegetation at the edge of the plot (Nye and Greenland 1960, p. 88). The danger comes as population pressure causes an increasing proportion of the land area to be cultivated for extended periods, or large-scale mechanical cultivation is introduced (Lal 1974) so that soil fertility and the vegetative cover are reduced, and accelerated erosion can take place if careful precautions are not taken.
Accelerated Erosion: The most widely practiced method of calculating the water erosion hazard is by the use of the Universal Soil Loss Equation, which was developed by Wischmeier and his associates in the United States (Wischmeier and Smith 1978). This method combines information on the erosivity of the rainfall in a particular area with the erodibility of the soils, together with land and crop management factors to calculate the predicted soil loss. Vegetation or Crop Cover: Roose (1977) has reported on many years of work on measuring erosion in francophone West Africa. For example, he measured erosion losses on a ferrallitic soil at Adiopodoumé, near Abidjan, Côte d'Ivoire, with an annual rainfall of 2138 mm (Table 3). One of the characteristics of erosion is its extreme variability depending on the particular conditions prevailing (Table 3). These data, with results quoted by Charreau (1974), indicate that the annual soil losses from cultivated fields under a wide range of conditions varied from about 0.1–138 t/ha: the higher value being more than 1300 times the lower. It is significant that the maximum soil loss that is considered tolerable in the United States is 11.2 t/ha per year (Hudson 1981). Table 3. Annual soil loss by water erosion at Adiopodoume, Côte d'Ivoire. Crops Slope (%) Soil loss (t/ha per year) Range Mean Cassava and yams Maize Groundnuts Bare soil Bare soil Bare soil 7 7 7 7 4.5 20–23.3 22–93 35–131 59–120 69–150 34–74 266–622 32 92 82 138 60 570 Source: Roose (1977, table 14).
In general, the extent of erosion is inversely proportional to the vegetative cover over the soil during the rainy season, whether in the form of natural vegetation, perennial crops, one or more annual crops, or leaf litter or mulch on the soil surface. It follows from this that perhaps the most important farming practices for the prevention of erosion are those that maintain an effective vegetative cover over the soil for the maximum time during the rainy seasons. This is considered in more detail in the next chapter, but at this point the importance of early planting of annual crops cannot be over stressed.
Soil Type and Structure: The erodibility of savanna soils, or their vulnerability to erosion, is generally high because many of the soils are sandy, low in OM, and of unstable crumb structure (Kowal and Kassam 1978, p. 168). It appears that certain types of fine sandy loam or loamy sand soils that are low in OM through overcultivation, and therefore have unstable structures, are particularly liable to erosion because of their tendency to crust. When a high intensity rainstorm hits bare soil, the fine soil particles are loosened by rain splash and are washed down into the pore spaces, which quickly become blocked. Once the pore spaces are blocked, and the soil surface is saturated, runoff starts (Chase and Boudouresque 1989). The fine OM and lighter soil particles are the first to be removed by runoff: measurements show that the eroded material contains two to four times more of the fine particles containing nutrients than the original soil (Jones and Wild 1975, p. 65; Kowal and Kassam 1978, p. 171).
On these soils, when the surface soil dries, the crust can become so hard that germinating seedlings of some crops cannot penetrate it. Also, further heavy storms can cause severe erosion because the crust causes most of the rain to run off instead of infiltrating into the soil (Ker et al. 1978, p. 39; Jones 1987, p. 12). On the other hand, the coarse dune sands that are found in parts of the Sahelian and Sahara zones appear to have such high infiltration rates that little water erosion takes place. Instead, these soils are liable to erosion by wind (this is discussed later). Some of the soils of volcanic origin that occur in parts of East Africa appear to have such a stable crumb structure that their erodibility also seems to be comparatively low, except under the most severe storms.
Angle and Length of Slope: The amount and speed of runoff depend on the steepness of the slope. The scouring capacity of running water increases as about the fifth power of its velocity (Jones and Wild 1975, p. 62). Therefore, slow-moving runoff may cause little erosion, although it can reduce the amount of soil moisture available for plant growth. As the runoff picks up speed, it causes subrill erosion, which is the most widespread form of erosion damage, but is often difficult for the untrained observer to identify. This type of erosion gradually removes the most fertile topsoil, until only infertile subsoil remains. It is particularly serious where a shallow topsoil overlies an iron pan or concretionary layer. In some parts of Africa, all the topsoil has been removed by erosion, leaving massive plinthite, which is useless for agriculture, or ironstone gravel, which is almost as bad. As runoff accelerates down the slope, it becomes concentrated into small rivulets, which cause rill erosion. This is easier to observe than subrill erosion and, on long slopes with inadequate protection, the water may eventually form gullies, which can cause severe damage.
Wind Erosion: Like water erosion, soil erosion by wind is only serious where there is no protective cover of vegetation. Wind erosion is particularly severe in the Sahara and Kalahari deserts and in the subdesert fringes (for example, the northern Sahelian Zone in West Africa), although it can occur elsewhere. Although experimental evidence is lacking, there appears little doubt that an increase in wind erosion has occurred in these desert and subdesert areas as a result of the destruction of much of the natural vegetation. This destruction is mainly due to overgrazing by excessive numbers of domestic livestock. Damage to the tree cover has also been caused by the clearing of bush for increased cultivation and for firewood. Wind erosion seems to be mainly confined to the regions receiving less than about 500 mm mean annual rainfall because, in the higher rainfall areas, there is usually enough vegetation — sometimes in the form of crops — to protect the soil from serious wind damage (Roose 1989).
During the dry season in West Africa, the northeast trade wind — the Harmattan — often carries a cloud of dust that covers much of West Africa, even as far south as the Cameroon and Nigerian coast. There have been reports that West African dust has been identified as far from Africa as the West Indies and parts of Europe. Although this consequence of wind erosion may have occurred from ancient times, some observers consider that the quantities of dust carried in recent years may have increased, particularly during drought periods; however, few quantitative measurements seem to be available. Hudson (1981) has reviewed the subject of wind erosion in some detail.
Desertification: The Sahelian drought of 1968–73 aroused world-wide concern about the possible spread of the Sahara desert into the Sahel region, and perhaps further southward into the Sudanian region. This concern was extended to other parts of the world that were similarly affected. The United Nations conference on desertification, held in Nairobi in 1977, both attempted to summarize some of the available information on the subject and had the effect of arousing widespread interest. Many widely varying estimates of the rate of spread of the Sahara have been published, but these do not seem to be derived from rigorous scientific studies. The arid area certainly fluctuates according to the rainfall, and particularly during a run of dry years such as occurred from 1968 to 1973, and again from 1979 to 1984, the arid area with its accompanying wind erosion extended southward. However, it is not clear what happens during a run of wetter years. Certainly, the vegetation makes some recovery in these drought-affected areas and, because the livestock population may have been reduced during the dry period by migration southward or by deaths, the vegetation has a chance to reestablish itself. However, when the herders move the livestock northward again and the herds regain their former numbers by breeding, the grazing pressure on the vegetation may again become heavy and the next dry period may lead to further damage, which may eventually become irreversible. Current indications of global warming may also have substantial effects both on temperature and rainfall in Africa, but it seems too early to predict probable outcomes.
Soil Conservation: Because soil conservation must be an integral part of farming systems, it is considered in the next chapter on farming systems. Copyright 1995 - 2003 © International Development Research Centre fromhttp://web.idrc.ca/en/ev-31004-201-1-DO_TOPIC.html
AN ASSESSMENT OF THE SOIL RESOURCES OF AFRICA IN RELATION TO PRODUCTIVITYhttp://www.nrcs.usda.gov/technical/worldsoils/papers/africa2.html
Below a brief analysis of the environmental problems facing savanna Africa. The most important being land use, rainfall, and soils (see Chapter 2).
African soils are as varied as those in other parts of the world, but because many of them are derived from the ancient granitic rocks of the African shield over much of the continent their inherent fertility is low. Sanchez (1976, p. 73) estimated that alfisols and oxisols alone cover 1100 million ha or 55% of the total land area. Most of these soils are sandy with low CECs and limited nutrient contents, often shallow over ironstone gravel or plinthite, and subject to erosion. Although soil OM contents can be quite high under natural vegetation or forest, they decline rapidly when the land is cleared and cultivated. Acid soils are widespread, and although some crops can tolerate certain levels of acidity, the growth and yields of many important crops are reduced. Many savanna countries have natural deposits of lime and phosphatic rock, but these deposits have been little used by smallholders. As land pressure increases because of the growth of human and livestock populations, more and more land is denuded of its natural vegetation by cultivation, overgrazing, and the removal of trees. Eventually, some cultivators are forced onto hill slopes and other marginal lands. The increasing proportion of the land that is left bare and exposed to high-intensity rainfall is prone to erosion unless urgent counter-measures are taken.
The dangers of soil erosion are outlined in Chapter 2, and soil-conservation measures considered in the discussion of many of the farming systems in Part II. However, there is no doubt that the battle to conserve the soils of Africa is being lost. This is obvious to anyone flying over such countries as Ethiopia, where the severe scarring of hillsides and the muddy state of rivers and streams indicate the enormous losses of topsoil that are occurring. Yet Ethiopian farmers have developed their own simple soil-conservation methods. For example, after preparing the land for a crop with the traditional ard (a plow without a mouldboard), farmers will always finish the work with a few shallow furrows across the field, about 10–15 m apart, just off the contour, so that any runoff will run down these furrows and off the field instead of continuing on down the field. However, the heavy population pressure in most of the highland areas forces people to cultivate any fallow land available, including progressively steeper hillsides. Also, the repeated cultivations necessary to prepare land for the staple grain, teff, during the early part of the rainy season mean that over-cultivated land is exposed to heavy rains, so the traditional contour furrows, although reasonably effective with a lower population density, do not prevent erosion.
It is well known that erosion is negligible under natural vegetation undisturbed by farmers or their domestic animals. In the areas with more favourable rainfall distribution — where perennial crops such as bananas, cocoa, coffee, tea, or fruit trees could be grown — provided a good leaf canopy was maintained over the soil, little erosion would take place, and farmers often developed their own methods of conserving rainfall with ditches and basins. Mulching in bananas and other crops was also an effective soil conservation method. Many farmers were also skilled in intercropping young perennial crops with other crops such as beans or groundnuts that reduced weed competition and covered the soil. They would also allow "soft" weeds, such as Galensoga parviflora, that did not compete strongly with the crops to grow during the rains, to be slashed and left as a mulch or dug in as a green manure during the dry season. In many traditional shifting-cultivation systems, farmers have developed ways of keeping a cover on the soil for as long as possible, either with natural vegetation, weeds, or crops. Grain crops are often planted as early as possible after the start of the rains, after a rough digging or plowing, both to give the best possible chance of obtaining a good crop, and so that they would have grown enough to cover the soil before the heavy rains, which often come about the middle of the rainy season. Mixed cropping with fast-growing crops such as beans or cowpeas is also commonly practiced, and these not only give an early crop, but also help to cover the soil. With increasing pressure on the land, however, these methods often become less effective. This is particularly apparent on the larger continuously cultivated plots on the lighter soils, where soil OM is reduced, soil structure deteriorates, and crop growth is poor allowing unimpeded erosion to take place, especially on the steeper slopes.
In some countries, colonial governments tried to impose various soil conservation measures on the farmers, mainly in the form of contour grass strips, contour "bunds" (banks), and narrow- and broad-based terraces (Hudson 1981). These were often resented by the farmers, particularly where land was limited, and sometimes they were destroyed after independence. In Kenya, a Swedish-funded soil-conservation project first did a careful survey of indigenous soil-conservation methods, such as the fanya juu contour ditches, and then trained government extension workers in these methods. The extension workers were then able to train the farmers in the same methods, with good results in many areas (Eriksson et al. 1980). guest (Read)(Ottawa) Login Important Notices | Copyright 1995 - 2003 © International Development Research Centre at http://web.idrc.ca/en/ev-32912-201-1-DO_TOPIC.html
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Soil Map West Africahttp://www.devecol.org/devecolsite/GeoElinks/Africa/TropicalSouth/Level2/TropSouth_Angola_soils.htm#
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